August 1, 2011

In the Linked Data context, a node of the Web of data is redirected to the document containing its description. “Data” (or “datum”, if you’re pedantic) as the basic unit of this new Web of data, represents a new paradigm that needs to take over the role that the “document” had before. However, this idea is not fully elaborated, and documents still exist as data containers. In other words, the data structure is “glued” onto the (2D) document instead of being implemented via (3D) HTTP URIs.

Creating the Web of data is a challenge because an RDF graph and the Web graph are two different types of graphs. Arcs (links) in an RDF graph have names, while hyperlinks on the Web have only direction. Another problem is the fact that in an RDF graph, out of three types of nodes only URI references are identified by an URI. How to follow the idea of ​​Web documents and assign each “data” on the Web of data a URI, when blank nodes and literals have no URI? On the Web of documents, every document has a URI, there are no “blank documents” and “literal documents”.

July 27, 2011

Check out the first post of the Problems of Linked Data series. The subject of the post is the identity of Linked Data:

In mid-2009 Paul Miller asked the question Does Linked Data need RDF?. He stated that the idea that Linked Data can only be Linked Data if expressed in RDF is a dogmatism that makes him „deeply uncomfortable“. A big debate questioning (once basic) assumptions of Linked Data began, and there is still no consensus today.

The problem arose because of the imprecise definition of the Linked Data rules that can be interpreted in different ways. The document that defines Linked Data and its rules is a personal note by Tim Berners-Lee and hasn’t been formally approved by the W3C. Even the term “Linked Data” itself is controversial and contributes to the confusion in the sense that the exact concept of “Linked Data” is conflated with the general idea of linking (any) data. Anyway, the third rule for publishing Linked Data has led to the most confusion and debate:

July 21, 2011

An RDF syntax (notation) is a concrete syntax for writing (serializing) RDF triples. There are many different notations, some of which are based on existing formats (XML, (X)HTML, JSON), while others use special formats (Turtle).

The problems of the RDF model, discussed in the previous posts, are inherited by RDF notations. Various special cases require special support in a syntax, which makes it more complex. However, RDF notations contain a number of their own problems, which can be divided into two categories:

July 19, 2011

In the previous posts I’ve covered two important problems of the RDF model, related to blank nodes and literals. Here, I’m going to focus on what I think is the key problem of RDF – the problem of the node of an RDF graph.

July 14, 2011

Check out the new blog post on my new (personal) blog. It’s the first post of the series on the results of my recent research, regarding the realisation of the Web of data/the Semantic Web. The first part is about the problems of the RDF model and the subject of this post are blank nodes:

Nodes without a name represent a special kind of nodes called blank nodes (bNode). These nodes simply indicate the existence of a thing, without using, or saying anything about, the name of that thing. Therefore, they are referred to as existential variables of an RDF graph.

Due to the absence of a name (URI), manipulating data containing blank nodes is much harder – they make otherwise trivial operations far more complex. They complicate the lives of data consumers, especially if data changes in the future. Blank nodes add a lot of complexity to the standards built upon them, and the implementations consuming them. They are poorly understood and difficult for beginners.

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